Reaction mechanisms, SNAr leaving groups

Sn2 and SNAr Reactions In these reactions the metal atom attacks aliphatic or aromatic carbon bonded to X, respectively. A stronger nucleophilic metal as well as a better leaving group X (I>Br>Cl>F) facilitates, whereas steric hindrance in R slows these types of oxidative addition [193, 194]. SNAr reactions are favored by electron-withdrawing substituents Y in the case of the substrates 4-YQH4X [2], Sn2 [27, 29, 89, 117, 180, 181] and SNAr [31, 33, 62-67, 95, 100, 107-109] mechanisms have been suggested frequently for zerovalent d10 complexes such as [L M] (M = Ni, Pd, Pt L=tertiary phosphine =2,3,4). For example ... 

Chromium tricarbonyl-complexed aryl fluorides undergo nucleophilic substitution reactions. The substitution is not a straightforward SNAr mechanism as can be seen using, for example, 4-methoxy-l-fluorobenzene complex (71). Reaction of (71) with acetylide (72) gives a 1 2 mixture of the 1,2 and 1,3 products (73) and (74) (Scheme 115). Other leaving groups include halogens, alkoxides, and amines. Indazoles can be prepared by reaction with hydrazine followed by acidic deprotection-decomplexation (Scheme 116). 

The basic concepts of nucleophilic substitution reactions appeared in the first semester of organic chemistry. These reactions follow SN1 or SN2 mechanisms. (In aromatic nucleophilic substitution mechanism, we use the designation SNAr.) In SN1 and SN2 mechanisms, a nucleophile attacks the organic species and substitutes for a leaving group. In aromatic systems, the same concepts remain applicable, but with some differences that result from the inherent stability of aromatic systems. 

The influence of steric effects on the SNAr mechanism has been studied by means of leaving group fluorine kinetic isotope effects. The reaction of DNFB with o-and -toluidines shows a change in the rate-determining step depending on the steric hindrance of the amine employed. 

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